The apparatus and process of the present invention relates generally to infrared detector elements, and more specifically to the fabrication techniques used to achieve closely packed one- or two-dimensional photoconductive mercury cadmium telluride infrared detector arrays.
Mercury cadmium telluride (HgCdTe) detectors are commonly used in scanning infrared optical systems. These detectors are constructed in a variety of configurations, for example in front-side illuminated photoconductive arrays or in backside illuminated photodiode arrays. In the past, these system designs have typically required close spacing of the detector elements in only one dimension.
Close spacing has been accomplished in front-side illuminated photoconductive arrays by detector fabrication techniques which use photoresist to define the detector edges in combination with either acid etching or ion beam milling to remove the extraneous detector material. In some systems, close spacing is required in two directions. However, in these cases it is desirable to use approaches that are as similar to those used in one dimensional arrays as possible.
For example, in one well-known fabrication technique, i.e., the "Z" technology, individual detector elements are formed (usually with nearly standard processing) on the edges of thin substrates which are then placed side by side to achieve close packing. Prior to stacking the arrays side by side, the detector electrode material is selectively deposited over the detector, epoxy and substrate edges, thus providing a detector electrical contact and defining the detector active area.
However, close spacing as required by recent optical system designs (i.e., 0.001 to 0.002 inches between detector active areas) are difficult to achieve using the Z technology. First, the Z technology requires a certain minimum detector spacing in order to bring the detector electrode up over the etched or milled substrate/detector edge. Second, this requires cutting through the glue bond between detector and substrate which invariably results in erosion of the detector active area. Finally, the process of contacting the detector to the underlying circuit board conductive pad requires additional space between detectors. These three aspects of the Z technology (bringing the contact up over the detector edge, glue cutting and contacting) require so much space between detectors that linear packing densities greater than ninety percent (90%) are difficult to achieve using detector elements placed 0.010 inch center-to-center.
In photodiode backside illuminated HgCdTe arrays, two-dimensional packing of detectors has been more easily accomplished, but performance has suffered as a result of the fabrication technique used. In one fabrication technique, a window glass material, such as IRTRAN manufactured by Eastman Kodak Co., is etched or milled to form a checkerboard pattern of raised square plateaus, where each plateau is separated by an etched area on all four sides. A metalization is deposited in the etched areas such that transmission is limited to the raised areas.
This window is then epoxied to a HgCdTe detector substrate which is subsequently lapped, polished, masked and doped to form a photodiode array. The detector active areas are formed over the plateaus of the window in order to maximize optical transmission to the detectors through the epoxy. While both close packing and masking are achieved with this technique, transmission is reduced unpredictably by the thickness of epoxy between the raised plateaus and the HgCdTe substrate.
It is, accordingly, a primary object of the present invention to provide an apparatus and method for fabricating a backside illuminated detector array which may be used to construct detector array assemblies which are closely packed in either one or both directions, and have better transmission and performance characteristics.